BF: Dietary Factors, Breast Milk and Infant Outcomes (2008)


Motil KJ, Thotathuchery M, Bahar A, Montandon CM.  Marginal dietary protein restriction reduced nonprotein nitrogen, but not protein nitrogen, components of human milk. J Am Coll Nutr. 1995; 14 (2): 184-191. 

PubMed ID: 7790694
Study Design:
non-randomized controlled trial
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To determine whether a short-term marginal reduction of dietary protein intake altered milk production or reduced milk's protein, nitrogen (N) or amino acid concentration in well-nourished lactating women.

To estimate whether total daily production of the nitrogenous components of milk may be compromised. 

Inclusion Criteria:
  • Healthy, lactating women
  • 21 years to 38 years of age
  • One, five or 12 months postpartum
  • Well-nourished
  • Non-smoking
  • Not taking any medications
  • Parity of three or fewer
  • Uncomplicated pregnancy
  • Delivered appropriate-for-gestational age, term infant. 
Exclusion Criteria:

Not reported.

Description of Study Protocol:


Not reported.


Non-randomized controlled trial (short-term).

Blinding used

Not reported.


  • Habitual protein intake (1.5g per kg per day) for seven days vs. marginal protein intake (1.0g per kg per day) for 10 days
  • Compliance: During controlled-diet period, all women instructed to weigh all foods/beverages; consumed four isocaloric and isonitrogenous meals per day. 

Statistical Analysis

  • Descriptive statistics
  • Multiple regression analysis to examine differences over postpartum period between two groups
  • Two-sample T-tests to determine differences between two groups for any group×time interactions between independent variables.
Data Collection Summary:

Timing of Measurements

Initial data collection outside of lab: At-home, three-day food record to determine usual food intakes. The habitual protein group was given a controlled constant diet approximating usual intakes of 1.5g protein per kg per day for seven days. Marginal protein group was given controlled protein diet of 1.0g protein per kg per day for 10 days (to ensure complete adaptation to lower protein level). 

Food intake over 72 hours measured and recorded by each subject throughout three consecutive days, including one weekend day. Usual intakes calculated from these records.   

Data collection in lab: During last four days of dietary period, subjects admitted to lab for measurement of N-balance, milk production and composition and total daily production of N components of milk.

Dependent Variables

  • Milk production (test-weighing of infants; manual/mechanical pumping; milk consumed by infant; milk expressed by mother)
  • Milk composition (from entire contents of alternate breasts for 24h): N and non-pro N (automated microKjeldahl method); Energy (adiabatic bomb calorimetry).

Independent Variables

  • Protein intake (including total N):  habitual protein intake (1.5g per kg per day) for seven days vs. marginal protein intake (1.0g per kg per day) for 10 days
  • Time postpartum.

Control Variables

Controlled dietary intake; timing of measurements.

Description of Actual Data Sample:


Initial N: 24

Attrition (final N): 24 [12 in habitual-protein (HP) group and 12 in marginal-protein (MP) group]

Age: 27.0y±3.2y and 29.8y±4.5y in HP and MP groups, respectively.

Ethnicity: Not reported.

Other relevant demographics: Not reported.


  • Height 164.9cm±6.9cm for HP group and 167.5cm±2.8cm for MP group
  • Weight 64.8kg±17.5kg for HP group and 63.8kg±5.6kg for MP group
  • BMI 23.8±6.1 for HP group and 22.7±1.7 for MP group.

Usual energy intakes: 37kcal±11kcal per kg per day for HP group and 35kcal±7kcal per kg per day for MP group.

Location: Houston, TX.


Summary of Results:



Habitual protein (HP)


Marginal protein (MP)


Statistical Significance of Group Difference

Protein intake (g/kg/d)




Milk production(g/d)




Milk total N (umol/g)




Milk production, milk total-N, and N-components (protein N, urea, lactoferrin, free and bound amino acids) significantly decreased over postpartum time (P<0.01), though remained similar between two protein groups. 


Author Conclusion:

Maternal milk production and the protein N (but not the non-protein N) fraction of human milk were relatively well-preserved in the presence of a short-term, marginal reduction of dietary protein. 

More prolonged and severe restrictions of maternal dietary protein intake may diminish the production of selected N-components of milk and adversely affect the nutritional well-being of the breast-fed infant.

Funding Source:
Government: USDA/ARS
University/Hospital: Baylor College, Texas Children Hospital
Reviewer Comments:

Women, though "assigned" to different levels of protein (HP=1.5g/kg/d; MP=1.0g/kg/d), reported consuming the same amount of protein (1.5 g/kg/d). 

Researchers report that duration of diet assignment marginal protein group (1.0g/kg for 10 days) vs. seven days for habitual protein group was " insure that metabolic adaptation to lower level of dietary protein was relatively complete."


Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) ???
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? No
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? No
  2.2. Were criteria applied equally to all study groups? ???
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? No
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) No
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? N/A
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? No
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? ???
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? ???
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? No
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? ???
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? N/A
  7.5. Was the measurement of effect at an appropriate level of precision? ???
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? ???
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes